Backpropagation, or reverse-mode automatic differentiation, is handled by the Flux.Tracker module.

julia> using Flux.Tracker

Here we discuss some more advanced uses of this module, as well as covering its internals.

Taking Gradients

In the basics section we covered basic usage of the gradient function.

using Flux.Tracker

Tracker.gradient((a, b) -> a*b, 2, 3) # (3.0 (tracked), 2.0 (tracked))

gradient is actually just a thin wrapper around the backpropagator-based interface, forward.

using Flux.Tracker: forward

y, back = forward((a, b) -> a*b, 2, 3) # (6.0 (tracked), Flux.Tracker.#9)

back(1) # (3.0 (tracked), 2.0 (tracked))

The forward function returns two results. The first, y, is the original value of the function (perhaps with tracking applied). The second, back, is a new function which, given a sensitivity, returns the sensitivity of the inputs to forward (we call this a "backpropagator"). One use of this interface is to provide custom sensitivities when outputs are not scalar.

julia> y, back = forward((a, b) -> a.*b, [1,2,3],[4,5,6])
(param([4.0, 10.0, 18.0]), Flux.Tracker.#9)

julia> back([1,1,1])
(param([4.0, 5.0, 6.0]), param([1.0, 2.0, 3.0]))

We can also take gradients in-place. This can be useful if you only care about first-order gradients.

a, b = param(2), param(3)

c = a*b # 6.0 (tracked)


Tracker.grad(a), Tracker.grad(b) # (3.0, 2.0)

Tracked Arrays

The param function converts a normal Julia array into a new object that, while behaving like an array, tracks extra information that allows us to calculate derivatives. For example, say we multiply two parameters:

julia> W = param([1 2; 3 4])
Tracked 2×2 Array{Float64,2}:
 1.0  2.0
 3.0  4.0

julia> x = param([5, 6])
Tracked 2-element Array{Float64,1}:

julia> y = W*x
Tracked 2-element Array{Float64,1}:

The output y is also a TrackedArray object. We can now backpropagate sensitivities to W and x via the back! function, and see the gradients accumulated in the W and x tracked arrays:

julia> Tracker.back!(y, [1, -1])

julia> W.grad
2×2 Array{Float64,2}:
 5.0   6.0
-5.0  -6.0

julia> x.grad
2-element Array{Float64,1}:

You may sometimes want to drop derivative information and just get the plain value back. You can do this by calling

Custom Gradients

We can hook in to the processes above to implement custom gradients for a function or kernel. For a toy example, imagine a custom implementation of minus:

minus(a, b) = a - b

Firstly, we must tell the tracker system to stop when it sees a call to minus, and record it. We can do this using dispatch:

using Flux.Tracker: TrackedReal, track, @grad

minus(a::TrackedArray, b::TrackedArray) = Tracker.track(minus, a, b)

track takes care of building a new Tracked object and recording the operation on the tape. We just need to provide a gradient definition.

@grad function minus(a, b)
  return minus(data(a),data(b)), Δ -> (Δ, -Δ)

This is essentially just a way of overloading the forward function we saw above. We strip tracking from a and b so that we are calling the original definition of minus (otherwise, we'd just try to track the call again and hit an infinite regress).

Note that in the backpropagator we don't call data(a); we do in fact want to track this, since nest AD will take a derivative through the backpropagator itself. For example, the gradient of * might look like this.

@grad a * b = data(a)*data(b), Δ -> (Δ*b, a*Δ)

For multi-argument functions with custom gradients, you likely want to catch not just minus(::TrackedArray, ::TrackedArray) but also minus(::Array, TrackedArray) and so on. To do so, just define those extra signatures as needed:

minus(a::AbstractArray, b::TrackedArray) = Tracker.track(minus, a, b)
minus(a::TrackedArray, b::AbstractArray) = Tracker.track(minus, a, b)

Tracked Internals

All Tracked* objects (TrackedArray, TrackedReal) are light wrappers around the Tracked type, which you can access via the .tracker field.

julia> x.tracker
Flux.Tracker.Tracked{Array{Float64,1}}(0x00000000, Flux.Tracker.Call{Void,Tuple{}}(nothing, ()), true, [5.0, 6.0], [-2.0, -2.0])

The Tracker stores the gradient of a given object, which we've seen before.

julia> x.tracker.grad
2-element Array{Float64,1}:

The tracker also contains a Call object, which simply represents a function call that was made at some point during the forward pass. For example, the + call would look like this:

julia> Tracker.Call(+, 1, 2)
Flux.Tracker.Call{Base.#+,Tuple{Int64,Int64}}(+, (1, 2))

In the case of the y we produced above, we can see that it stores the call that produced it – that is, W*x.

julia> y.tracker.f
Flux.Tracker.Call{...}(*, (param([1.0 2.0; 3.0 4.0]), param([5.0, 6.0])))

Notice that because the arguments to the call may also be tracked arrays, storing their own calls, this means that Tracker ends up forming a data structure that records everything that happened during the forward pass (often known as a tape).

When we call back!(y, [1, -1]), the sensitivities [1, -1] simply get forwarded to y's call (*), effectively calling

Tracker.back(*, [1, -1], W, x)

which in turn calculates the sensitivities of the arguments (W and x) and back-propagates through their calls. This is recursive, so it will walk the entire program graph and propagate gradients to the original model parameters.